The Wheel of Time series is not the same as most of the other series mentioned here. It is a serial - a single story told in 14 volumes - rather than a series of stand-alone stories featuring the same characters.

I can't off-hand think of any other serial of comparable length, although there are plenty of much longer series.

One that immediately springs to mind is Patrick O'Brian's "Aubrey/Maturin" naval series, which is often described as a single novel in 20 (plus one unfinished) volumes.